The present invention is in the field of tensile testing machines. More particularly, the present invention provides an improved apparatus and method for measuring the peel strength of a circuit line or film bonded to a substrate such as a circuit board. Also, the present invention relates to universal testing machines providing tensile, compressive, shear, bending, and torsion tests on a material sample.
Tensile testing machines are commonplace. Typically, a material test sample is clamped to a horizontal platform, and a clamping device attached to a force gauge is lowered in a vertical direction and attached to the test sample. Means are provided to move the clamping device and force gauge in an upward direction, thereby causing a tensile force to be applied to the test sample.
Commonly, samples are destructively tested in a large tensile testing machine, wherein a sample to be tested must be sacrificially extracted from a larger specimen for the sake of the test. Once the test has been completed, the sample is usually discarded, which can be prove to be very costly.
Typical tensile testing machines used for laminate bond testing only provide peeling movement in one direction, so that the electrical circuit line or other sample on a substrate being tested must be lined up along the one direction of movement. Unfortunately, this requires the substrate to be repositioned and clamped whenever a circuit line has a directional orientation different from the previous test direction. Further, when peeling a circuit line from a substrate, the force gauge must be constantly moved in order to keep the force gauge directly over the peeling location. This is necessary in order to ensure that a true force reading of the force perpendicular to the substrate is being measured.
The present invention avoids the disadvantages of the prior art by providing a compact mini-tension tester. The mini-tension tester includes a base plate, a x-axis slide apparatus, a y-axis slide apparatus, a z-axis slide apparatus, a servo actuator assembly, a force gauge, a cable, and a gripper clamp.
A substrate with a film or circuit line bonded to its surface can be attached to the base plate using clamps, vacuum means, or other attaching systems. The z-axis slide apparatus is slidably attached to the y-axis slide apparatus and the y-axis slide apparatus is slidable attached to the x-axis slide apparatus. Air bearings, or other frictionless type mechanisms, are used to provide essentially friction free motion. Therefore, the z-axis slide apparatus can move essentially friction free to any location within the x and y plane.
A servo actuator assembly is attached to the z-axis slide apparatus, and a force gauge is attached to the servo actuator assembly. The force gauge preferably comprises a strain gauge load cell, although other types of force measurement devices may be used. A cable connects the force gauge to a gripper clamp that is attached to a test sample located on a substrate. The test sample may include, for example, a circuit line or film formed on a substrate such as a printed circuit board.
In order to measure the pull force required to pull a circuit line or film from a substrate, the substrate is firmly attached to the platform using a vacuum system. In order to obtain test data without destroying the substrate or affecting the operation of the circuitry on the substrate, sample test circuit lines are applied to the substrate during the production process. Preferably, the sample test circuit lines are only used to monitor the production process, and are not involved with any part of the electronic functioning of circuitry on the substrate. Therefore, these circuit lines may be peeled off the substrate for testing, without sacrificing the operational circuitry on the substrate.
During testing, the end of a circuit line is peeled from the substrate and grasped by the gripper clamp. Next, a servo actuator assembly in the z-axis slide apparatus displaces the force gauge, cable, and gripper clamp upward in the z-direction at a constant velocity, thereby providing an upward force that peels the circuit line away from the substrate. The servo actuator assembly is force limited to provide a maximum of about 20 pounds of force. The desired force reading is the force applied in a direction perpendicular to the substrate. In the present invention, the friction free air bearings in the x-axis slide apparatus and y-axis slide apparatus allow the z-axis slide apparatus to xe2x80x9cwalkxe2x80x9d with the circuit line release or peel point. This ensures that the force applied to the release point of the circuit line is always perpendicular to the substrate. Therefore, the force gauge is always measuring the desired force, that is, the force perpendicular to the substrate.
The use of the mini-tension tester is not restricted to only providing tensile testing, but can also provide compressive, shear and bending material testing, and strength testing. For instance, compressive testing can be conducted by providing a rigid member between the force gauge and the test object. Then the force cell is moved along the z-axis direction toward the test object, thereby creating a compressive force on the test object. For applying shear force, a test object can be clamped onto the base plate in a direction such that the desired shear force is in line with the z-axis of the mini-tension tester. In another embodiment, a shear force can be applied to the test object by attaching the servo actuator assembly in a direction perpendicular to the z-axis. For this case, a shear force can be applied to a test object in a direction parallel to the base plate. If a test object is attached to the base plate in a cantilevered manner, the servo actuator assembly can apply a force in the z-axis direction to the free end of the cantilever causing a bending moment in the test object.
The present invention additionally provides a mini-tension tester that is compact enough to fit inside an oven to provide elevated temperature testing. The mini-tension tester is portable and versatile since a variety of substrate sizes can be attached to the base plate. Also, the mini-tension tester is much less costly then the large tensile testing machines that it replaces.
Generally, the present invention provides an apparatus for measuring the peel strength of a material bonded to a substrate, comprising:
a gripper clamp for grasping a material bonded to a surface of a substrate;
a force gauge attached to a z-axis displacement system and coupled to the gripper clamp, wherein a displacement of the z-axis displacement system causes the material to peel away from the substrate; and
x and y-axis displacement systems attached to the z-axis displacement system for providing self-aligning orthogonal positioning of the force gauge relative to a release point of the material as the material is peeled away from the substrate.